This invention relates to a dual drive web feed apparatus and method for intermittent moving of a web and particularly for positioning a web in a web working machine such as bag forming machine.
In the formation of successive similar elements from a web of paper, film or other material, a continuously moving web may be intermittently moved to locate successive lengths of the material through a forming machine in a time stepped manner. A particularly satisfactorily intermittent feed drive is disclosed in the above identified application of the inventor. The above application particularly discloses a drive having a programmed logic controller for producing a very controlled intermittent direct drive of the web. The web feed mechanism includes a draw roll drive with the programmed logic controller which creates a drive profile for each web feed cycle to provide a controlled acceleration of the drive means to a maximum drive speed followed by a deceleration of the drive means to a stop position within the available drive time, thereby providing for a controlled movement of the web. The programmed controller as disclosed therein processes the necessary input signals based on a previous cycle to establish a motion profile of the motor servo drive during each drive period and establishes a highly effective and efficient actuation of the drive means within the total time available to move the web.
In various applications, a precise length of material is to be moved each step and a controlled tension is desired in the web during the working of the web. Various applications may require a constant high, low intermediate tension. In order to create a controlled tension system, an upstream drive and a downstream web drive such as individually driven sets of draw rolls may be coupled to the web and operated in synchronism to simultaneously move the web through the unit and hold the web in place during processing with the required tension on the web. This requires accurate synchronized movement of the draw rolls and coupling to the web to provide precise stepping of the web while maintaining the desired tension. Various prior art systems generally similar to those used in controlling a single draw roll drive have been suggested. Thus, in a known system, two sets of draw rolls are coupled to the web and an electromagnetic clutch and brake system controls the master or upstream draw rolls. A variable ratio cone and pulley drive couples the downstream drive rolls to the upstream drive. A variable ratio setting, controlled either manually or by a dancer unit, changes the drive ratio slightly to maintain a constant length of web in each intermittent stepped drive. Although such systems have been used, the systems have rather significant commercial implications and limitations as to cost, accuracy and repeatability. Thus, the mechanical components used have a relatively high inertia thereby limiting the response and speed of correction. In addition, the systems present relatively costly maintenance, both as to down time and parts cost associated with such apparatus.
There is therefore a very significant need and demand for an improved drive system which can provide a rapid response to any change in the web length and preferably which will avoid the high maintenance cost associated with the prior art mechanical drive modification.